FIG. 1 is a cross sectional view of a conventional permanent magnet DC motor. The motor comprise a stator and a rotor 3 rotatably mounted confronting the stator. The stator comprises a cylindrical housing 1, permanent magnets 2 fixed at inner surface of the cylindrical housing. The motor will probably roll in transportation due to its cylindrical housing, and is inconvenient to install.
A square motor is showed schematically in FIG. 2. The motor also comprises a stator and a rotor 3′. The stator uses a square motor housing 1′, which comprises four side portions and four corner portions. Permanent magnets 2′ are fixed on an inner surface of the side portions and distributed around the rotor 3′, which is rotatably mounted confronting the stator. The square motor solves the above-mentioned drawbacks. However, the square motor has a low space utilization rate, and the space near the corner portions is not used. Furthermore, the outer diameter of the rotor is restricted by the size of the square housing and the thickness of the magnets, especially in a small motor.